Method for delivering atrial defibrillation therapy

ABSTRACT

An apparatus and method for delivering electrical shock therapy in order to treat atrial tachyarrhythmias such as fibrillation is disclosed. In accordance with the method, atrial defibrillation shocks are delivered synchronously with an R wave if the current R-R interval meets one or more safety criteria so as to be considered shockable. A shockable R-R interval may be defined as one that exceeds the previous QT interval by a specified therapy margin. In one embodiment, the previous QT interval is estimated based upon the measured preceding R-R interval.

CROSS-REFERENCE TO RELATED APPLICATION(S)

[0001] This patent application is a continuation of U.S. patentapplication Ser. No. 09/661,875, filed on Sep. 14, 2000, thespecification of which is incorporated herein by reference.

FIELD OF THE INVENTION

[0002] This invention pertains to methods for treating atrialtachyarrhythmias. In particular, the invention relates to an apparatusand method for delivering shock therapy to terminate atrialfibrillation.

BACKGROUND

[0003] Tachyarrhythmias are abnormal heart rhythms characterized by arapid heart rate, typically expressed in units of beats per minute(bpm). They can occur in either chamber of the heart (i.e., ventriclesor atria) or both. Examples of tachyarrhythmias include sinustachycardia, ventricular tachycardia, ventricular fibrillation (VF),atrial tachycardia, and atrial fibrillation (AF). Tachycardia ischaracterized by a rapid rate, either due to an ectopic excitatory focusor abnormal excitation by normal pacemaker tissue. Fibrillation occurswhen the chamber depolarizes in a chaotic fashion with abnormaldepolarization waveforms as reflected by an EKG.

[0004] An electrical shock applied to a heart chamber (i.e.,defibrillation or cardioversion) can be used to terminate mosttachyarrhythmias by depolarizing excitable myocardium, which therebyprolongs refractoriness, interrupts reentrant circuits, and dischargesexcitatory foci. Implantable cardioverter/defibrillators (ICDs) providethis kind of therapy by delivering a shock pulse to the heart whenfibrillation is detected by the device. An ICD is a computerized devicecontaining a pulse generator that is usually implanted into the chest orabdominal wall. Electrodes connected by leads to the ICD are placed onthe heart, or passed transvenously into the heart, to sense cardiacactivity and to conduct the shock pulses from the pulse generator. ICDscan be designed to treat either atrial or ventricular tachyarrhythmias,or both, and may also incorporate cardiac pacing functionality.

[0005] The most dangerous tachyarrythmias are ventricular tachycardiaand ventricular fibrillation, and ICDs have most commonly been appliedin the treatment of those conditions. ICDs are also capable, however, ofdetecting atrial tachyarrhythmias, such as atrial fibrillation andatrial flutter, and delivering a shock pulse to the atria in order toterminate the arrhythmia. Although not immediately life-threatening, itis important to treat atrial fibrillation for several reasons. First,atrial fibrillation is associated with a loss of atrio-ventricularsynchrony which can be hemodynamically compromising and cause suchsymptoms as dyspnea, fatigue, vertigo, and angina. Atrial fibrillationcan also predispose to strokes resulting from emboli forming in the leftatrium. Although drug therapy and/or in-hospital cardioversion areacceptable treatment modalities for atrial fibrillation, ICDs configuredto treat atrial fibrillation offer a number of advantages to certainpatients, including convenience and greater efficacy.

[0006] As aforesaid, an ICD terminates atrial fibrillation by deliveringa shock pulse to electrodes disposed in or near the atria. The resultingdepolarization also spreads to the ventricles, however, and there is arisk that such an atrial shock pulse can actually induce ventricularfibrillation, a condition much worse than atrial fibrillation. To lessenthis risk, current ICDs delay delivering an atrial shock pulse until theintrinsic ventricular rhythm is below a specified maximum rate and thendeliver the shock synchronously with a sensed ventricular depolarization(i.e., an R wave). That is, a current R-R interval, which is the timebetween a presently sensed R wave and the preceding R wave, is measured.If the current R-R interval is above a specified minimum value, theinterval is considered shockable and the atrial defibrillation shockpulse is delivered.

[0007] Judging a current R-R interval to be shockable or not basedsolely upon whether it exceeds a single specified minimum value,however, can lead to errors because the period during which theventricle is vulnerable to fibrillation may not be reflected by thecurrent R-R interval. For example, certain R-R interval sequences, suchas a long-short R-R interval sequence, are particularly dangerous forshock timing which thus increases the risk of fibrillation for a givenspecified minimum interal. In order to lessen this risk, the specifiedminimum interval value can be increased, but this has the effect ofdelaying the delivery of atrial defibrillation therapy until thepatient's heart rate drops to a rate corresponding to the increasedminimum interval value. It is an objective of the present invention toprovide an improved method for detecting shockable R-R intervals so asto allow defibrillation shocks to be delivered in a safe and timelymanner.

SUMMARY OF THE INVENTION

[0008] The present invention is a method and apparatus for deliveringatrial defibrillation therapy in which delivery of an atrialdefibrillation shock pulse is delivered synchronously with a sensedR-wave if the current R-R interval meets one or more safety criteria soas to be considered shockable. A first criterion defines a shockable R-Rinterval as one that exceeds the previous QT interval by a specifiedtherapy margin. The previous QT interval may be determined by detectinga T-wave following an R-wave or estimated as a function of the measuredpreceding R-R interval. A second criterion may be applied that requires,in addition to meeting the first criterion, that a current R-R intervalbe longer than a specified minimum interval value in order to beconsidered shockable. A third criterion may also be applied whichconsiders a current R-R interval shockable if it exceeds a specifiedsufficiently-long interval value irrespective of the length of thepreceding R-R interval, where the sufficiently-long interval is longerthan the specified minimum interval value.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a diagram illustrating criteria for determining ashockable R-R interval.

[0010]FIG. 2 is a system diagram of an implantable defibrillator.

DETAILED DESCRIPTION OF THE INVENTION

[0011] The present invention is a method and apparatus for deliveringatrial defibrillation shock therapy. As used herein, atrialdefibrillation shock therapy should be taken to mean shock therapy fortreating any atrial tachyarrhythmia, such as atrial flutter, as well asatrial fibrillation.

[0012] In order to avoid the possible induction of ventricularfibrillation, conventional ICDs deliver atrial defibrillation shockssynchronously with a sensed R wave and after a minimum pre-shock R-Rinterval. (The R-R interval is the time between the immediatelypreceding R wave and the presently sensed R wave, and an R wave may beregarded as either a spontaneously occurring depolarization or aventricular pace.) This is done because the ventricle is especiallyvulnerable to induction of fibrillation by a depolarizing shockdelivered at a time too near the end of the preceding ventricularcontraction (i.e., close to the time of ventricular repolarization asindicated by a T wave on an EKG). Delivering the shock synchronouslywith a sensed R wave thus moves the shock away from the vulnerableperiod. At a rapid ventricular rhythm, however, the ventricular beatsmay be so close together that even a synchronously delivered shock mayinduce ventricular fibrillation. A minimum pre-shock R-R interval istherefore employed to provide a safety margin. Relying solely on thecurrent R-R interval to determine if an R-wave is safe to shock on,however, does not take into account the variability in the length of thevulnerable period due to variations in the length of the QT interval ofthe preceding beat. This may lead both to shocks being delivered duringthe vulnerable period and to unnecessary delays in delivering shocks.

[0013] In accordance with the present invention, one or more criteriaare employed to achieve greater precision in defining a shockable R-Rinterval than with previous methods. A first criterion is to define acurrent R-R interval as shockable if it exceeds the QT interval of theprevious beat by a specified therapy margin. The QT interval may bemeasured either by detecting the T-wave of the previous beat or byestimating it as a function of the previous R-R interval. In the lattercase, the QT interval as a function of the preceding R-R interval,QT_((previous RR)), may be estimated as a linear function of theprevious R-R interval:

QT_((previous RR))=A (R-R_(prev))

[0014] where A is a defined constant and R-R_(prev) is the measuredpreceding R-R interval. A more accurate calculation, however, is to usea logarithmic formula of the following form:

QT _((previous RR)) =K ln (R-R _(prev))−C

[0015] where K and C are defined constants. A QT interval calculated bythis formula has been found to correlate well with measured QT intervalsin normal human subjects with K and C set to 166.2 and 715.5,respectively. In subjects with prolonged QT intervals due to Class IIIantiarrhythmic drugs, bundle branch block, or other disorders, however,it has been found that a more accurate estimate of the QT interval isgiven by setting K and C to 185.5 and 812.3, respectively. As these arethe types of patients for whom implantation of an ICD is typicallyindicated (i.e., because they are at risk for sudden cardiac death),this is the presently preferred formula for estimating the QT intervalin ICD patients. The criterion for judging whether a current R-Rinterval is safe to shock on then becomes:

R-R interval>185.5 ln (R-R _(prev))−812.3+TM

[0016] where TM is a specified therapy margin (e.g., 60 ms). Thiscriterion thus effectively excludes R-R intervals that are part of along-short interval sequence from being considered shockable.

[0017] A minimum R-R interval criterion may also be employed in additionto the QT interval therapy margin described above. In this embodiment, acurrent R-R interval is considered shockable if it exceeds the previousQT interval by a specified therapy margin TM and exceeds a specifiedminimum interval MI. The combined criteria for determining shockabilityof an R-R interval may then be stated as:

R-R interval>185.5 ln (R-R _(prev))−812.3+TM

AND

R-R interval>MI

[0018] A third criterion may also be employed that overrides the QTinterval criterion if the current R-R interval is sufficiently long. Inthis embodiment, an R-R interval is considered shockable if it exceeds aspecified sufficiently-long interval SL regardless of the length of theprevious R-R interval. The combination of all three criteria may then bestated as:

((R-R interval>185.5 ln (R-R _(prev))−812.3+TM) AND (R-R interval>MI))

OR

(R-R interval>SL)

[0019] where SL is greater than MI.

[0020]FIG. 1 graphically illustrates the combination of the threecriteria by means of a Poincare map. The vertical axis represents theprevious R-R interval, while the horizontal axis represents the currentR-R interval. Points on the right and left sides of the criterion lineCL are considered in the shockable and non-shockable domains,respectively. Thus a current R-R interval will be considered shockableif the previous R-R interval is such that the point lies to the right ofthe criterion line CL. The criterion line is divided into threesegments, labeled CL1 through CL3, which represent the three criteriafor judging the shockability of an R-R interval described above. The CL1segment is part of a vertical line corresponding to the equation:

current R-R interval=MI

[0021] The CL2 segment is part of a curve corresponding to the equation:

current R-R interval=K ln (R-R _(prev))−C+TM

[0022] where MI is the specified minimum interval, TM is the specifiedtherapy margin, and K and C are specified constants for the logarithmicequation that estimates a QT interval from the previous R-R interval. Inanother embodiment, the CL2 segment is a straight line with a specifiedslope. The CL3 segment is part of a vertical line corresponding to theequation:

current R-R interval=SL

[0023] where SL is the specified sufficiently-long interval. Thus for ashort previous R-R interval that estimates a short QT interval, thecriterion for shockability is dictated by segment CL1 so that only acurrent R-R interval that exceeds MI is considered shockable. Only whenthe previous R-R interval becomes long enough so that the sum of theestimated QT interval and the therapy margin TM exceeds MI does segmentCL2 come into play in determining shockability. For previous R-Rintervals that fall within the CL2 segment, a current R-R interval isconsidered shockable only if it exceeds the sum of the estimated QTinterval and the therapy margin. When the previous R-R interval is longenough so that the sum of the estimated QT interval and the therapymargin TM exceeds the sufficiently-long interval SL, shockability isdetermined solely by whether or not the current R-R interval exceeds SLas represented by the segment CL3.

[0024]FIG. 2 is a system diagram of a microprocessor-based implantablecardioverter/defibrillator device for treating atrial tachyarrhythmiasthat in which the method described above may be implemented. In thisdevice, which also includes a pacemaker functionality, a microprocessorand associated circuitry make up the controller, enabling it to outputpacing or shock pulses in response to sensed events and lapsed timeintervals. The microprocessor 10 communicates with a memory 12 via abidirectional data bus. The memory 12 typically comprises a ROM or RAMfor program storage and a RAM for data storage. The ICD has atrialsensing and pacing channels comprising electrode 34, lead 33, sensingamplifier 31, pulse generator 32, and an atrial channel interface 30which communicates bidirectionally with a port of microprocessor 10. Theventricular sensing and pacing channels similarly comprise electrode 24,lead 23, sensing amplifier 21, pulse generator 22, and a ventricularchannel interface 20. For each channel, the same lead and electrode areused for both sensing and pacing. The sensing channels are used tocontrol pacing and for measuring heart rate in order to detecttachyarrythmias such as fibrillation. The ICD detects an atrialtachyarrhythmia, for example, by measuring the atrial rate as well aspossibly performing other processing on data received from the atrialsensing channel. A shock pulse generator 50 is interfaced to themicroprocessor for delivering shock pulses to the atrium via a pair ofterminals 51 a and 51 b that are connected by defibrillation leads toshock electrodes placed in proximity to regions of the heart. Thedefibrillation leads have along their length electrically conductivecoils that act as electrodes for defibrillation stimuli. A similar shockpulse generator 60 and shock electrodes 61 a and 61 b are provided todeliver ventricular fibrillation therapy in the event of an inducedventricular fibrillation from atrial shock pulses.

[0025] The device in the figure also has the capability of measuring theelectrical impedance between electrodes 34 a and 34 b. A current isinjected between the electrodes from constant current source 43, and thevoltage between the electrodes is sensed and transmitted to theimpedance measurement interface 30 through sense amplifier 31. Theimpedance measurement interface processes the voltage signal to extractthe impedance information therefrom and communicates an impedance signalto the microprocessor. If the electrodes 34 a and 34 b are disposed inproximity to the heart, the impedance signal can be used to measurecardiac stroke volume. An example of this technique is described in U.S.Pat. No. 5,190,035, issued to Salo et al. and assigned to CardiacPacemakers, Inc., which is hereby incorporated by reference.

[0026] The device depicted in FIG. 2 can be configured to deliver atrialdefibrillation therapy in accordance with the invention as describedabove by appropriate programming of the microprocessor. Thus, once anepisode of atrial fibrillation is detected with the atrial sensingchannel, the device prepares to deliver an atrial defibrillation shock.The ventricular rhythm is monitored by measuring the R-R intervalassociated with each sensed R wave. An atrial defibrillation shock pulseis then delivered synchronously with a sensed R wave if a shockablecurrent R-R interval is measured, where a shockable current R-R intervalis defined as an interval that is longer than a preceding QT interval bya specified therapy margin, where the QT interval may be estimated fromthe previous R-R interval. If a minimum interval criterion is alsoimplemented, only if a sensed R wave also occurs at an R-R intervallonger than a specified minimum limit value is sensed R wave consideredsafe to shock on. If a sufficiently-long criterion is employed, acurrent R-R interval is considered shockable if it exceeds a specifiedsufficiently-long interval value irrespective of the length of thepreceding QT interval. The device may be programmed so as to specify anyof the defined constants that dictate the shockability criteria such asMI, TM, SL, K, and C. The shockability criteria may thus either be basedupon population data or tailored to the individual patient.

[0027] Because detected R-waves are used to calculate the R-R intervals,it is important for R-waves to be detected as accurately as possible anddistinguished from noise. In order to improve the reliability of R-wavesensing, the device of FIG. 2 may be further programmed to use theimpedance signal reflecting stroke volume as an indication ofventricular systole. When an R-wave is detected, only if an impedancesignal is also detected synchronously therewith is the R-wave consideredvalid and used to compute an R-R interval. In another embodiment,multiple ventricular electrodes can be used to sense R-waves. Forexample, two ventricular sensing channels may be used such that a sensedR-wave is considered valid only if it is sensed by both channels.Reliably sensed R-waves can also be used in where T-waves are sensed andused to determine QT intervals. In such embodiments, a reliably sensedR-wave can be used to aid in distinguishing a T-wave from an R-wave by,for example, subtracting the R-wave component from a sensed electrogramto leave only the T-wave component, or causing a T-wave detector toignore all detected events within a certain time interval before orafter a detected R-wave.

[0028] Although the invention has been described in conjunction with theforegoing specific embodiment, many alternatives, variations, andmodifications will be apparent to those of ordinary skill in the art.Such alternatives, variations, and modifications are intended to fallwithin the scope of the following appended claims.

What is claimed is:
 1. A system for delivering an atrial defibrillationshock pulse, the system comprising: an atrial sensing channel to detectatrial fibrillation; a ventricular sensing channel to sense ventriculardepolarizations (R waves); a shock pulse generator to generate theatrial defibrillation shock pulse; and a controller coupled to theatrial channel, the ventricular channel, and the shock pulse generator,the controller being programmed to: measure a first R-R intervalassociated with a first R-wave; estimate a first QT interval based onthe first R-R interval; measure a second R-R interval associated with asecond R-wave subsequent to the first R-wave; and deliver the atrialdefibrillation shock pulse synchronously with the second R-wave if thesecond R-R interval is longer than the first QT interval.
 2. The systemof claim 1, comprising an implantable cardioverter/defibrillator.
 3. Thesystem of claim 2, further comprising a pacemaker.
 4. The system ofclaim 1, wherein the controller is programmed to deliver the atrialdefibrillation shock pulse synchronously with the second R-wave if thesecond R-R interval is longer than the first QT interval by a specifiedtherapy margin.
 5. The system of claim 4, wherein the controller isprogrammed to deliver the atrial defibrillation shock pulsesynchronously with the second R-wave if: the second R-R interval islonger than the first QT interval by the specified therapy margin; andthe second R-R interval is longer than a specified minimum value.
 6. Thesystem of claim 4, wherein the specified therapy margin is approximately60 milliseconds.
 7. The system of claim 4, wherein the controller isprogrammed to deliver the atrial defibrillation shock pulsesynchronously with the second R-wave if the second R-R interval exceedsa specified sufficiently-long interval irrespective of the length of thefirst QT interval and the therapy margin.
 8. The system of claim 4,further comprising an impedance measurement circuit adapted to detect animpedance signal indicative of ventricular systole for validating theR-wave sensing.
 9. A method for delivering an atrial defibrillationshock pulse, the method comprising: detecting an episode of atrialfibrillation; sensing ventricular depolarizations (R waves); measuring afirst R-R interval associated with a first R-wave; estimating a first QTinterval based on the first R-R interval; measuring a second R-Rinterval associated with a second R-wave subsequent to the first R-wave;and delivering the atrial defibrillation shock pulse synchronously withthe second R-wave if the second R-R interval is longer than the first QTinterval.
 10. The method of claim 9, wherein delivering the atrialdefibrillation shock pulse comprises delivering the atrialdefibrillation shock pulse synchronously with the second R-wave if thesecond R-R interval is longer than the first QT interval by a specifiedtherapy margin.
 11. The method of claim 10, wherein delivering theatrial defibrillation shock pulse comprises delivering the atrialdefibrillation shock pulse synchronously with the second R-wave if: thesecond R-R interval is longer than the first QT interval by thespecified therapy margin; and the second R-R interval is longer than aspecified minimum value.
 12. The method of claim 11, wherein thespecified therapy margin is approximately 60 milliseconds.
 13. Themethod of claim 11, wherein delivering the atrial defibrillation shockpulse comprises delivering the atrial defibrillation shock pulsesynchronously with the second R-wave if the second R-R interval exceedsa specified sufficiently-long interval irrespective of the length of thefirst QT interval and the therapy margin.
 14. The method of claim 11,further comprising detecting an impedance signal indicative ofventricular systole to improve a reliability of the R-wave sensing. 15.A method for delivering cardiac defibrillation therapy, the methodcomprising: detecting fibrillation; sensing ventricular depolarizations(R waves); measuring R-R intervals each associated with one of the Rwaves; determining whether a current R-R interval exceeds a specifiedsufficiently-long interval (SL), the current R-R interval associatedwith a current R wave; delivering a defibrillation shock pulsesynchronously with the current R wave if the current R-R intervalexceeds the SL; and delivering the defibrillation shock pulsesynchronously with a current R wave if: the current R-R interval exceedsa specified minimum interval (MI); and the current R-R interval exceedsa value given as K ln (R-R_(prev))−C+TM, wherein K and C are each aspecified constant and TM is a specified therapy margin, and R-R_(prev)is a previous R-R interval associated with an R wave immediatelypreceding the current R-wave.
 16. The method of claim 15, furthercomprising detecting an impedance signal indicative of ventricularsystole, and wherein measuring R-R intervals each associated with one ofthe R-waves comprises measuring the R-R intervals each associated withthe one of the R-waves and an impedance signal detected synchronouslywith the one of the R-waves.
 17. The method of claim 15, furthercomprising determining values of MI, SL, K, C, and TM based uponpopulation data.
 18. The method of claim 15, further comprisingtailoring values of MI, SL, K, C, and TM to an individual patient. 19.The method of claim 15, further comprising determining values of MI, SL,K, C, and TM based upon at least one of population data and individualpatient data.
 20. The method of claim 19, wherein K and C areapproximately 166.2 and 715.5, respectively, and wherein the R-Rintervals are measured in milliseconds.
 21. The method of claim 19,wherein K and C are approximately 185.5 and 812.3, respectively, andwherein the R-R intervals are measured in milliseconds.
 22. The methodof claim 19, wherein TM is approximately 60 milliseconds.